Z-Axis or Metric Unit gives a worng size to any Object
Try popping out the properties panel (it is a panel in the 3d view on the right, usually summoned with the N keyboard shortcut).
Set this to the Item tab, and you can set dimensions there.

You may need or want to reset the scales to 1 in all axes.  If you like the shape of your object but still want to reset the scale you can Apply Scale (ctrl-A is default shortcut for apply menu).  As you can see from the screenshot the dimensions are different from the scale and from the location.
If you set the location first, and then the dimensions, the scale will be set appropriately for you.